1. Field of the Invention
The present invention relates to a color image forming apparatus of the called multi-transfer type in which the toner images of color components are successively transferred onto a print sheet put on an image transfer member in a superposed fashion. More particularly, the invention relates to a color image forming apparatus of the type in which a plural number of developing units, which contain toner of different colors respectively assigned thereto, are successively set at a position facing an image carrier member one by one.
2. Discussion of the Conventional Art
A known color image forming apparatus of the called multi-transfer type includes a photoreceptor drum on which toner images are formed in accordance with image data, and an image transfer drum with a print sheet wound thereon. The photoreceptor drum is turned in synchronism with the image transfer drum. The toner images of different colors are successively formed on the photoreceptor drum being turned. These toner images are transferred onto the print sheet put on the image transfer drum in superposed fashion. The thus superposed toner image is fixed on the print sheet by a fixing unit.
In the color image forming apparatus, a plural number of developing units, which contain toner of different color components (e.g., black K, yellow Y, magenta M and cyan C) respectively assigned thereto, are disposed around the photoreceptor drum. In operation, one of the developing units is moved to a developing position facing the photoreceptor drum, and develops a latent electrostatic image on the photoreceptor drum. At this time, the remaining developing units are placed apart from the photoreceptor drum. To form a color image, those developing units are successively moved to the developing position one by one every time the developing process for the toner image of one color is completed. Accordingly, any of the developing units is always positioned at the developing position when the color image forming apparatus is placed in a developing mode.
When the color image forming apparatus is not in the image formation mode, namely, it is in a stand-by mode in preparation for the incoming image forming operation, all of the developing units are placed apart form the developing position. When the image forming operation starts, the developing unit for the first toner image is moved to the developing position.
In the color image forming apparatus of the multi-transfer type, the toner image is transferred from the photoreceptor drum to a print sheet on the image transfer drum every turn of the image transfer drum. The lengths of an image forming section and a nonimage forming section are determined by only the outside diameter of the image transfer drum and the size of the print sheet used. In connection with this, a specific example is given. When a print sheet of A3 size (of JIS (Japanese Industrial Standards) is used (moved longitudinally for image formation), the required circumferential length of the image transfer drum is at least 420 mm. To form an image by moving laterally a print sheet of A4 size (of JIS) by using the same image transfer drum, the required length of the image forming section is 210 mm, and then the length of the nonimage forming section is 210 mm or more. The result is that the nonimage forming section is longer than the image forming section.
This causes the following problems. In the color image forming apparatus constructed as mentioned above, after the image forming operation starts, any of the developing units is always located at the developing position. The time that the nonimage forming section on the photoreceptor drum faces the developing unit is inevitably long. As a result, the photoreceptor drum is soiled with toner from the developing unit. The toner transferred to the photoreceptor drum is further transferred to the image transfer drum. When a print sheet is put on the image transfer drum, the print sheet is also soiled with the toner.
The color image forming apparatus has edit functions, such as a color conversion and an image insertion composition. As known, the image insertion composition is an image editing process for forming a composite image by inserting a specified image on an original document into an image on another original. When using the edit function, image data is processed during a period of time between the formation of a toner image of a first color and the formation of a toner image of a second color. The data processing time must be integer times as long as the time of one turn of the image transfer drum. Accordingly, the nonimage forming section is correspondingly lengthened. The time that the nonimage forming section faces the developing unit is also elongated correspondingly. As a result, the soiling problem is more remarkable.
A technique that handles print sheets of large heat capacity, such as sheets for an overhead projector (OHP) and thick sheets, in the color image forming apparatus is disclosed in Unexamined Japanese Patent Publication 4-337751. In the publication, to give a satisfactory amount of heat energy to the print sheet, the drum speed of the image transfer drum is reduced after the transferring operation of the toner image of the final color onto the print sheet is completed. The print sheet bearing the toner image thereon is moved through the fixing unit at the reduced speed. Accordingly, the drum speed of the image transfer drum is increased and decreased during a period of time between the formation of an toner image of the final color and the formation of an toner image of the next first color. The speed changing time must also be integer times as long as the time of one turn of the image transfer drum. Accordingly, also in this case, the nonimage forming section on the photoreceptor drum is correspondingly lengthened, and the time that the photoreceptor drum faces the developing unit is elongated. The soiling problem remains unsolved.
In a case where a monocolor image, not a color image, is successively formed on a plural number of print sheets, a specific developing unit is continuously set at the developing position for a period of time from the start and the end of the image forming operation. Accordingly, the time that the photoreceptor drum faces the developing unit is elongated. The soiling problem is still left unsolved.
Also in the case of forming a full color image, the developing units are successively moved to the developing position immediately after one developing operation is completed. Any of the developing units is placed at the developing position, excepting the changing of the developing unit. As a result, the developing unit faces the image transfer drum for a long time. The apparatus suffers from the same problem.